1. Field of the Invention
This invention relates in general to methods of making electret transducers and in particular to a method of making a backplate assembly for an electret transducer.
2. Description of the Prior Art
Electret transducers have been utilized in microphones and earphones and a common form utilizes a metallized thin plastic diaphragm which is supported in tension adjacent a conductive backplate. The diaphragm has a permanent charge on it so that no external D.C. bias is required for such electret transducers.
Diaphragms made of polytetrafluoroethylene or fluorinated ethylene-propylene copolymers produce stable electrets, however, the mass of such materials is so large that electret tranducers with such diaphragms have a narrower frequency response than conventional condenser transducers utilizing a diaphragm made of titanium or a metallized diaphragm made of polyethylene terephthalate (T. M. Myler). Thus good results have been obtained with a conventional diaphragm mounted close to a backplate assembly which has a conductive surface and an electret film attached to the conductive surface. The electret film is secured to the conductive surface by conventional adhesives and the bond between the electret film and conductive surface has been poor in prior art devices. The bonding between the electret film and the conductive surface has been particularly poor after being charged at high temperatures such as 100.degree. C. and after the electret transducers have been used for a period of time.
Thus, such electret transducers of the prior art have not been used in practice.